The present invention relates to a probe which is used in an NMR imaging apparatus or the like and, more particularly, to a slotted tube resonator type probe which is used in an NMR apparatus of a relatively low magnetic field.
Since the S/N ratio in the NMR apparatus increases in proportion to about 1.5 power of a static magnetic field H, there has been made a trial to improve the S/N ratio by increasing the magnetic field strength as large as possible. As a probe to transmit and receive a radio frequency signal of the NMR apparatus, a solenoid coil or a saddle-shaped coil has been used so far. However, since the resonance frequency also becomes high with an increase in magnetic field strength, there occurs a situation such that the self resonance frequency of the coil and the NMR frequency approach or are reversed. Thus, there occurs a problem such that the sensitivity upon reception deteriorates or the generating efficiency of the radio frequency magnetic field upon transmission deteriorates.
On the other hand, a coil having a new shape called "Alderman-Grant type" has been proposed by Alderman et al. The Alderman-Grant type coil has been disclosed in "Journal of Magnetic Resonance", Vol. 36, pages 447 to 451.
The Alderman-Grant type coil is fairly effective with respect to a point such as to solve the problem such that the sensitivity upon reception deteriorates due to the approach or reversal of the self resonance frequency of the coil and the NMR frequency due to the increase in resonance frequency by the increase in magnetic field strength mentioned above or the generating efficiency of the radio frequency magnetic field upon transmission deteriorates. In addition, the Alderman-Grant type coil has a feature such that the sensitivity distribution is more uniform as compared with that of the saddle-shaped coil or the like.
However, in the above related art, an attention is mainly paid to the improvement of the self resonance frequency and no consideration is made to the problem in the case where the feature of the uniformity of the sensitivity distribution or the like which the Alderman-Grant type coil has is contrarily applied to the low frequency band.
That is, since the self resonance frequency of the Alderman-Grant type coil is high, in order to apply the Alderman-Grant type coil to a low resonance frequency, it is necessary to increase the capacitance which is formed between the wing and the guard ring or to increase the capacitance which is inserted between the wings. To increase the capacitance which is formed between the wing and the guard ring, it is necessary to reduce the distance between the wing and the guard ring or to increase the areas of the wing and the guard ring.
However, when the distance between the wing and the guard ring is reduced, the distance between the electrodes decreases, so that the withstanding voltage decreases. On the other hand, there occurs a problem such that the coil characteristic also remarkably varies due to a slight change in the distance between the electrodes by the external force. In addition, when the areas of the wings and guard rings are increased, the whole probe becomes long, resulting in an increase in weight.
On the other hand, there is a problem such that when the capacitance which is inserted between the wings is set to be too large, the operation of the probe is not executed as a whole.
Although there is also considered a method of substantially increasing the capacitance between the wing and the guard ring by inserting the capacitor between the wing and the guard ring, there is a problem such that the characteristic of the probe deteriorates.